Optical discs are widely used as information recording mediums capable of recording a large capacity of information. Along with the progress of the technology, optical discs having a higher recording density have been developed.
The optical discs which were first commonly used are compact discs (CDs), and then digital versatile discs (DVDs) have become common. DVDs can record information at a recording density about six times the recording density of CDs. Since a large capacity of data can be recorded on one DVD, DVDs are especially used for recording video information having a large information amount. Recently, optical discs capable of recording information at a still higher recording density, for example, HD-DVDs and Blu-ray discs (BDs), have been developed and have begun to be used especially for recording high precision video information.
As various types of optical discs are developed, compatibility among optical disc apparatuses becomes important. In consideration of the convenience for users, it is preferable that optical disc apparatuses are compatible to a plurality of types of optical discs.
As optical pickups for an optical disc apparatus compatible to a plurality of types of optical discs having different recording densities, optical pickups including a plurality of objective lenses are proposed as shown in, for example, Patent Documents 1 through 3. Such conventional optical pickups are mainly compatible to CDs and DVDs.
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-11765
Patent Document 2: Japanese Laid-Open Patent Publication No. 11-120587
Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-245650